The Beasts of Marseilles

Many films have ambiguous titles, but why "Seven Thunders"? Was this the title of the book from which this film was derived? Can anyone explain? Did I miss something in this slow and ponderous film? The whole storyline did not ring true. Where were the escapees going to from Marseilles? Spain would seem the obvious choice. Was this in fact an actual escape route for Allied POWs from Italy? The film was interesting from the fact that a lot of it was shot on location, but overall it was a very disappointing use of a talented cast! Interesting to see Stephen Boyd in an early screen role. Sad that his career appeared to peter out and that he died young.